404 - Underdrain
It is very difficult to determine the extent and volume of groundwater infiltration. Experienced Region Soils personnel usually make this determination. The inspector should make sure that the Contractor makes the installation in accordance with the specifications and the intent of the Soils personnel, so that complete interception and collection of groundwater is accomplished.
All dimensions, notes and details included in the Standard Plan R-80 series should be closely followed to assure that the underdrains and outlets function properly.
Subgrade underdrains are slotted or perforated drains placed near the edge of the traveled way for the purpose of lowering a high groundwater condition or draining a granular subbase. Bank underdrains are similar drains placed in cut slopes, or behind the slope stake line of cuts, for the purpose of intercepting seepage and preventing the weeping and sloughing of the slope. Open-graded underdrains serve a dual function of rapidly draining infiltrated water from the pavement structure, both under the pavement and at the subgrade-subbase contact point. The Standard Plan R-80 series details the cross section elevation of these drains.
Underdrains are designed to allow water to enter the pipe to conduct it away without loss of surrounding soils. Silting and plugging is one of the principle causes of failure. For this reason, it is very important that, when a granular backfill material is called for in the underdrain trench, the underdrain be wrapped in geotextile. When open-graded materials, such as Class 34R, are called for in the trench, the underdrain itself should not be wrapped with a geotextile (although the trench will be lined with a geotextile). It is extremely important that geotextile trench wraps be properly placed so that no “windows” occur that would allow soil to bypass the wrap and be washed out through the underdrain.
The connection between the underdrain pipe and the underdrain outlet pipe should be greater than 90 degrees. This is to enable the use of a small diameter video camera for inspection. On projects where major installations of open-graded underdrain and underdrain outlets are taking place, a Special Provision for Open-Graded Underdrain and Open-Graded Underdrain Outlet Video Inspection will more than likely be included in the project’s proposal. The purpose of the video inspection is to identify any installation defects in the underdrain pipe/underdrain outlet pipe such as the following.
- Crushed pipe.
- Separated joints.
- Plugged underdrain and/or underdrain outlet pipe.
- Standing water exceeding half the pipe diameter for a distance of 25 feet (7.6 m) or more.
The video inspection is made after the mainline pavement is placed but before the shoulder paving is performed. The underdrain outlets, including outlet endings, must be completely installed prior to conducting the video inspection. Defects found must be excavated and repaired, and/or removed and replaced, via a method proposed by the Contractor and meeting the Engineer’s prior approval. The C&T Pavement Unit performs the video inspection of the underdrain pipe/underdrain outlets. They can also be contacted for exploration of installed drains that were placed on projects that did not include a special provision requiring this work (phone 517-322-5711). The video camera can travel over 500 feet (150 mm) into the installed underdrain (a distance greater than the maximum underdrain outlet spacing).
Other areas requiring special attention that, in the past, have been observed to be problems with open-graded underdrain installations include the following.
Placing geotextile separator over the open-graded underdrain trench (on projects where geotextile separator is called for between the sand subbase and open-graded drainage course). This does not allow the system to function correctly. The area above the trench should not be covered by the geotextile separator.
Improper connections made within the outlet pipe. If connections are required in the outlet pipe, they need to be watertight and the connectors need to be of the same material as the outlet pipe (e.g. rigid PVC outlet pipe needs connections made with rigid PVC connectors).
Improper size of concrete rings (when Concrete Ring Option for outlet endings is used). The Standard Plan R-80 series calls for 3 foot (900 mm) lengths for the concrete rings and 2 foot (600 mm) lengths have been incorrectly installed in the past.
Sags in the underdrain pipe just before it connects to the underdrain outlet pipe.
The inspector must measure and record trench dimensions to ensure that specification widths and depths are being obtained. Underdrains, edgedrains and bank drains are measured in place by length in feet (meters) along the centerline of the underdrain. Since it is difficult to verify length after installation, this measurement at the time of installation is the only verification of the pay quantity and becomes more important than measurement of some of the other items placed on a project.